Efficient, robust and practical self-referenced optical frequency combs require diode-pumped femto-second lasers which can provide high peak powers at high repetition rates, ideally in the multi-100-MHz region. Comb applications prefer higher repetition rate sources in order to achieve more power per mode. However, the need to implement self-referencing via octave-spanning super-continuum generation means that a compromise must be reached between repetition-rate and laser peak power.
Recently, the greater commercial availability of highly-doped Yb:fibre has extended the operation of femtosecond Yb:fibre ring lasers to 570 MHz [see T. Wilken et al “High Repetition Rate, Tunable Femtosecond Yb-fibre Laser,” in Conference on Lasers and Electro Optics (Optical Society of America, 2010), paper CFK2]. Such fibre lasers have been used for the generation of octave-spanning super-continua. However, a problem with known systems is that they rely on the use of further amplification to achieve octave-spanning super-continua at repetition rates above 166 MHz [see H. Hundertmark et al “Octave-spanning supercontinuum generated in SF6-glass PCF by a 1060 nm mode-locked fibre laser delivering 20 pJ per pulse,” Opt. Express 17, 1919 (2009), and I. Hartl, et al “GHz Yb-femtosecond-fibre laser frequency comb,” in Conference on Lasers and Electro Optics (Optical Society of America, 2009), paper CMN1].